This is why I have a major problem with this, presumably you have been playing as normal? You haven't suddenly deposited 10x your normal amount and played low risk bets on low risk games?
They are only supposed to do these checks if they suspect you could be money laundering. As long as they aren't submitting a SAR, then they can talk to you about it, no matter what some reps say. So if you ask why they suspect you of this, when your play hasn't changed, then they should tell you. It isn't against the law to do so.
However, for the likes of us, giving a casino you are an affiliate for, details of your income is clearly giving sensitive business information over to them, which you shouldn't have to do. I can see it causing problems too, not saying Casumo will, but I'll bet some do. For example, if you promote Casumo, and your affiliate earnings for the previous 6 months have been £500, £200, £300, £150, £600, £300, and you supply Casumo a bank statement showing you got £7000 from fullcream, £5000 from LeoVegas, £2500 from Coral, £3500 from gigaffiliates etc, then whats the betting some point in the future you get an email from Casumo affiliates saying 'we feel you aren't promoting us to the best of your ability, and therefore we are giving you 14 days notice of your account being terminated'. I also don't think the other casinos will be happy about you giving out information what they are paying you either, in fact I'm sure some have that business information mustn't be shared in their T&C's.
Casumo's affiliate terms state
(a) to use its best efforts to actively and effectively advertise, market and promote the Casumo Website as widely as possible in order to maximize the Parties’ benefits
So they could quite easily say you are in breach of that if they find out you are earning 10x as much from other affiliate programs.
And
11.2 The Affiliate agrees to avoid disclosure or unauthorized use of the Confidential Information to third persons or outside parties unless with the Company’s prior written consent.
That would include earnings as it is figures from within the affiliate panel.
I have asked 3 reps now what will happen if I blank out affiliate earnings, and not one has responded!